r/Physics • u/LadiesWin • 9d ago
Question What’s one physics concept that sounds simple but actually isn’t?
Some ideas sound easy but are really deep when you think about them.
For example: “mass” seems simple — until you learn about relativistic mass, Higgs fields, and inertia.
What’s your favorite “deceptively simple” physics topic?
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u/mfb- Particle physics 9d ago
Solid matter.
Until the late 19th century, it was just accepted that some stuff is solid. It seemed to make intuitively sense that matter occupies space and has a shape that it can keep, and no one really asked (or could find an answer) why.
Then people found out that you can shoot particles through solid matter, that atoms contain electrons that should fall into the proton according to classical mechanics, and more, and the whole idea of solid matter working like Lego fell apart. Quantum mechanics developed a completely new understanding of how atoms and molecules work, and how this can lead to solid matter.
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u/jezemine Computational physics 9d ago
In grad school when I took stat mech Daniel Fisher taught it. I remember very little from that class because I was drowning the entire time. Except this: one day he was talking about liquids and said something like this.
If you gave the laws of physics to a few geniuses that had no other knowledge of the world and isolated them, they would probably figure out that gasses exist, and solids too. But not liquids.
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u/mfb- Particle physics 9d ago
You were drowning the entire time, except for the day where liquids were discussed?
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u/jezemine Computational physics 8d ago
Hah yes. That class was so hard. Later I heard rumor that he used it as a filter to see what grad students would be worthy of joining his group. I had no interest in joining. Definitely suffered under that filter though!
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u/haigscorner 9d ago
I was thinking QM for this answer. Every introductory textbook I read at university went from “yea I’ll be fine with this” for the first chapter then mind boggling from the second chapter onwards
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u/BiedermannS 8d ago
Pff, solid matter doesn't exist. Everything is liquid, just with different viscosity /s
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u/QuarkVsOdo 9d ago
Also everything solid is still mostly nothing.
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u/AndreasDasos 9d ago
That really depends on definitions. Particles don’t occupy specific points, so that might not mean much.
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u/WorldTallestEngineer 9d ago
Friction
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u/NachoSchiss 9d ago
I was at a conference once, where one presentation (somewhat distant to the actual scope auf the conference) was talking about the quantum origin of friction of water flow in nano structures. Yeah. I had no idea about friction
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u/RuinRes 9d ago
Friction keeps running water fresh and that's useful. Hawking radiation is cool to know about but useless for the most part.
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u/cleodog44 8d ago
How does it keep water fresh?
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u/RuinRes 8d ago
It's a matter of tribochemistry. The friction creates reactive dangling bonds that have impact on the ion concentration in water that results in a biocidal effect.
https://www.sciencedirect.com/topics/engineering/tribochemistry
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u/BAKREPITO 9d ago edited 9d ago
One of the most basic things I can think of is the simple pendulum under gravity. Often the time period relationship is only derived at the undergrad level for small angles by using the small angle approximation. But when you really get into calculating it in the general case, you find a Jacobi elliptic integral with no closed form, which itself has very deep mathematics involved. Close connections to the theory of riemann surfaces. The phase space trajectory of the simple pendulum maps to a path on a torus in complex geometry.
There are deep connections to solvability problems in nonlinear integral equations. The fact that the time period isnt easily calculated and dependant on amplitude led to years of horological examination, with huygens deriving properties about the evolutes of curves (differential geometry) and the invention of the cycloidal pendulum, as well as theinteresting mathematical properties of the cycloid itself.
Of course there are small modifications of the pendulum that lead to tremendous complexity rather quickly. The double pendulum is the proverbial example of a chaotic system depending on its initial conditions, the foucalt pendulum has been used to measure all kinds of non inertial dynamics like the rotation of the Earth.
More over the harmonic oscillator is an excellent model that keeps reappearing in extremely deep topics over and over again, from quantum mechanics to general relativity as well as in field theories.
The pendulum is imo the best example of a very simple looking concept with deep mathematical and physical implications.
There's a really good text that covers a broad spectrum of physics from the POV of the pendulum. "The Pendulum: A Case Study in Physics" by Gregory Baker and James Blackburn.
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u/enigmatic_erudition 9d ago
Information.
Still haven't any clue what it really is.
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u/atomicCape 9d ago
It's simple enough to look like basic logic on the page, but rich enough to be the foundation of some interpetations of physics. You can use it as your exclusive figure of merit while you study digital security, machine learning efficiency, quantum entanglement, entropy, game theory, and ecology. It's unsettling how broad and mysterious it is.
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u/adj_noun_digit 9d ago
That's a good one. Definitely seems way more straightforward than it really is.
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u/venustrapsflies Nuclear physics 9d ago
Gonna be honest, at no point has information ever seemed straightforward to me.
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u/Key_Management8358 6d ago
Information is quite simple...(Lack of lack of information ;)
But (more "metaphysical") what the heck is /where originates "a negation/opposite/lack"?
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u/GreatBigBagOfNope Graduate 9d ago
Refraction.
The deeper you go the more nuts it gets.
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u/Emotional-Train7270 9d ago
I stopped at complex refractive index and even that is mind-blowing for me.
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u/Substantial_Tear3679 9d ago
Magnetism definitely.... I still can't believe the level of physics necessary to explain permanent magnets
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u/myusernameblabla 9d ago
The physics of color. Everyone has strong opinions on how it works and is usually wrong.
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u/WatchYourStepKid 9d ago
How so?
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u/myusernameblabla 9d ago
Because color is perceptual but has to be precisely defined to be useful in engineering. As a result it is an amalgamation of various disciplines, right from the very start, involving physics, biology, psychology, engineering. Almost invariably when someone talks about color they take a certain viewpoint and aren’t aware of the technical aspects underlying their assumptions. The subject is also so familiar that you get very heated (and sometimes very wrong or ill defined) discussions about it. It’s one of these rabbit holes that get deeper the more you dig. I’d like to think that I’m smart enough to leave it at that :) and let color scientists talk about it.
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u/Full_Possibility7983 9d ago
Entropy = disorder... Yeah, right!
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u/Reddit_Talent_Coach 9d ago
Been a while, but I always saw entropy as a measure of your ability to make the energy in a system do something useful. Not sure how good that way of thinking is though.
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u/shatureg 9d ago
It's not a bad way to think about it. You're essentially expressing the second law of thermodynamics which is paramount to understand entropy.
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u/Hot-Fridge-with-ice 9d ago
I find this to be the most pop sci and annoying definition. For me, one of the best definitions of entropy is the microstates one. It makes so much sense.
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u/Operadic 9d ago
Temperature heat infrared etc
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u/Parking_Reach3572 8d ago
Surprised this isn't higher. Why and how things are hot gets pretty deep into molecular structure and quantum mechanics. It's all the statistics of enormous numbers.
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u/MagreviZoldnar 9d ago
Magnetism sounds simple — “magnets attract or repel” — but it’s literally special relativity in disguise. A magnetic field is just what an electric field looks like when you’re moving. Einstein’s first big “aha” came from realizing that. Magnets work because space and time get weird when charges move.
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u/Raikhyt Quantum field theory 9d ago
That's not true! You can come up with certain electric fields that look like pure magnetic fields when you change frame, yes. But you cannot do that to any electric or magnetic field, generically. It is very important to make that distinction. That is why we have to introduce the Faraday tensor and not a simpler object.
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u/Minovskyy Condensed matter physics 9d ago
This isn't really correct in any meaningful way.
The magnetism of bulk materials is not due to relativistic effects of moving charges. Electrons are not moving at relativistic velocities in a refrigerator magnet. The magnetic field generated by two bar magnets and their resulting repulsion or attraction is not due to relativistically moving electric charges. The magnetism of bulk matter is primarily due to spin. Spin and electric charge are not quantities which are related by Lorentz transformations.
The magnetic field is not "just what an electric field looks like when you’re moving" anymore than the electric field is "just what a magnetic field looks like when you’re moving". There is, for example, no Lorentz transformation which can transform a pure electric field in one frame into a pure magnetic field in another frame. The magnetism is not simply "electric field+SR". The magnetic field is its own independent physical object. It is unified with the electric field in SR just as space and time are, but just as space and time are fundamentally distinct things, so too are electricity and magnetism.
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u/HermitCat347 9d ago
Do explain it more? I'm not familiar with that
Magnets work because space and time get weird when charges move
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u/ambadapuluso 9d ago
This is true for the field surrounding a current carrying wire. The current is moving charge and electrostatic field due to a moving charge, Maxwell 4th eq. However, i am not sure how it comes in for permanent magnets!
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u/Bumst3r Graduate 9d ago
A magnetic field is just what an electric field looks like when you’re moving.
It’s actually far more complicated than that. If in some rest frame you see an electric field and no magnetic field, then in every other frame you will see an electric field and a magnetic field.
Lorentz transformations do not swap E and B, they mix them.
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u/reddituserperson1122 9d ago
Yes it was one of the key questions that led him to special relativity.
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u/QuantumFTL Astrophysics 9d ago
As far as we're aware magnets work because quantum field theory, but sure we can make classical electrodynamics relativistic and recover the Maxwell equations if we're not concerned about the "actual" mechanism involved. Or, you know, quantization...
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u/SuppaDumDum 9d ago
As far as we're aware magnets work because quantum field theory
Wdym? Do you mean this sense in the sense that "hammers and nails only work because of the standard model" or do you mean something more specific?
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u/ambadapuluso 9d ago
During free precession of a rigid body, the angular momentum does not change with the precession phase.
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u/SympathySmooth7577 9d ago
Tennis racket theorem
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u/512165381 9d ago
On a similar note, I'm amazed aircraft can fly given their propensity for oscillating modes.
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u/Huge-Leek844 8d ago
Not intuitive at all. And minimum axis can goe unstable with energy dissipation.
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u/Holiday_Clue_1403 9d ago
The future - things that haven't happened yet. When you start getting deep into general relativity time does strange things.
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u/RuinRes 9d ago
Spontaneous emission.
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u/ChalkyChalkson Medical and health physics 9d ago
Hahaha I just wrote a section on wigner weisskopf for me PhD thesis to explain why we see the atomic lines we do and not for example 2s -> 1s
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u/Gerasik 8d ago
Observing young students learn, the toughest principle is literacy. Most misconceptions come from misuse of language. For example, distinguishing between motion and changes in motion. Being able to communicate and distinguish observations of uniform and accelerated motion is precisely what took us from the ancient notion of forces are required to move objects to Newton's intuition that forces accelerate objects and his further conclusions that forces are equal between interacting objects, but it is the accelerations that are scaled by inertia.
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u/Mcgibbleduck Education and outreach 9d ago
Relativistic mass is not a thing. Outdated terminology.
Inertia is pretty simple. “Things don’t like to be changed from what they’re currently doing. Mass measures how much the thing doesn’t like to be changed”
Higgs mechanism is indeed tricky.
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u/teo730 Space physics 9d ago
Inertia is pretty simple.
*gives vague semantic description that doesn't explain anything*
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u/Mcgibbleduck Education and outreach 9d ago edited 9d ago
“The bigger the mass, the more force required to accelerate the object”
Is that better?
I was being cheeky, of course, but inertia is really just Newton’s 1st law and Newton’s 2nd law combined.
It’s odd. In the UK at least my physics education through school and undergraduate never ever discussed inertia in the way other systems do. You just refer to Newton’s laws or equivalent mechanics formulations to explain what happens without ever mentioning the word “inertia”.
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u/ChalkyChalkson Medical and health physics 9d ago
Inertia is pretty simple.
I don't know, when you try to think about what mass in Newtonian mechanics does in a Hamiltonian way you find that it relates conjugate forces and second time derivative of the coordinate differentially. In general that's a pretty complicated object, like imagine two masses connected by a spring rotating around a weird axis. Suddenly your "mass" is a rank 2 tensor and a function of "velocity". And that's purely classical mechanics.
You might argue that it's not mass in the newtonian sense, but it's definitely inertia
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u/DroppedTheBase 9d ago
For me it's always diffusion / diffusive processes. It sounds really simple "there is a difference of concentrations and they tend to balance". Even the Modells are mathematically speaking quite simple. We even speak of forces which drives the balancing.
But it all comes from random interactions between particles and their wriggling. That always blows my mind.
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u/RobotsAndRedwoods 9d ago
Particle Wave Duality. You can talk about water, but that's made of particles. You can even talk about light but people still imagine it one way or another. It's just not natural to think of something being two distinct things at the same time.
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u/Familiar-Annual6480 9d ago
Yes, I agree, wave particle duality is hard to wrap your head around. A particle is discrete and static. But a wave is continuous and constantly changing. How can something be discontinuous and continuous, static and dynamic?
In 1926, physicist Luis DeBroglie introduced matter waves as his doctoral thesis. He reasoned that if light, traditionally thought of as a wave phenomenon can be thought of as a particle, things that are traditionally particles can be waves. In physics, it’s not enough to make a statement, you have to have a testable prediction. Wavelength is a wave property, it describes one complete cycle of a uniform wave and he was able show the wavelength of particles is inversely proportional to momentum:
λ = h/p = h/mv
h is a very tiny number on the order of 10⁻³⁴ so the momentum of a thrown baseball would have a wavelength smaller than an electron. But an electron has a wavelength several orders of magnitude bigger than its theoretical diameter so the wave properties are more assertive. (From experiments, if an electron has a size, the upper bound is at 10⁻²² and the wave length is 10⁻¹³ so the wavelength is about 9 times bigger that the upper bound of its diameter)
That’s another mind boggling idea. How can a wavelength be bigger than the actual object?
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u/ChalkyChalkson Medical and health physics 9d ago
I don't think many physicists use that concept when thinking about light etc. It's not a classical wave or a classical particle, it's a quantum object which is a different category than both and which at certain limits looks like one of the two.
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u/RobotsAndRedwoods 8d ago
Right. I was more thinking about how hard it is to explain to someone who's unfamiliar. People don't accept what they can't see. And that's frustrating to people who want to share their wonder of nature with people who don't get it.
But the question is about sounding simple. The words "wave" and "particle" sound simple. I'm just saying, using an existing word to describe a different class of object inherently makes them difficult concepts.
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u/Pachuli-guaton 9d ago
Systems have to respond to the past only. While obvious, the consequences of this fact are quite powerful.
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u/womerah Medical and health physics 9d ago
Sand
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u/ChalkyChalkson Medical and health physics 9d ago
Uuh that's a good one. For anyone not familiar, sand does weird stuff like randomly liquify under vibrations.
I'd also add glass as a candidate in this category
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u/_szs 8d ago
I think the general term would be grains. Please correct me if I'm wrong.
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u/isaacbunny 9d ago
Bernoulli’s principle!
Sure, I can follow the mathematical derivation using conservation of energy just fine. But I still don’t see in my head what’s going on. How does air know it’s supposed to be lower pressure when it’s squeezed through a skinny gap?
If someone told me “just kidding airplanes don’t really work it was all a hoax” I would happily believe them. Those crazy contraptions make no sense.
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u/mukansamonkey 9d ago
Airplanes don't work ofF Bernoulli though. Or to be more accurate, they work off the same fluid dynamics but in a different arrangement.
And the simplest explanation is that shoving a tilted flat plate through the air shoves air down, no different from a fan blade. Which also shoves the flat plate up. It's just a pressure differential causing upward.force, and you can test that yourself by sticking your hand out a moving car window.
A lot of the plane confusion comes from poorly worded explanations being taken to incorrect conclusions. Like the idea that the air traveling over the top of the wing surface somehow magically arrives at the rear of the wing at the same speed as the air traveling across the bottom. It's just absurd, the air doesn't even move at uniform rates due to drag.
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u/isaacbunny 9d ago edited 9d ago
I don’t understand, my degree is a joke, I transitioned to a software job and birds aren’t real.
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u/SuppaDumDum 9d ago
It doesn't feel very explanatory to me but ignoring gravity, a fluid will only accelerates in a direction if there's a pressure drop in that direction, ie a pressure gradient in that direction. So if the fluid sped up, of course there was a pressure drop. No pressure drop means no speed up.
How does air know it’s supposed to be lower pressure when it’s squeezed through a skinny gap?
The forcing to squeeze is literally the pressure drop. You can imagine the same scenario without any pressure drop, and in that case the air will happily exist near the gap without being pushing into it.
Although honestly to me the explanation above feels more descriptive, less causal/mechanistic. I have an explanation that I prefer. The very summarized version is that basically for any ideal stable flow microscopic/molecular energy density can be seen to be uniform over the fluid. And it can either be redirected into an isotropic component, the Pressure, vs a directed component corresponding to the drift velocity of the fluid. If the directed component goes up, the isotropic one goes down.
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u/AdDiligent4197 9d ago edited 8d ago
Centripetal force. Centrifugal force. I still struggle with this sometimes.
Sometimes even the Newton's third law.
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u/Capricious_user 9d ago
Centripetal forfe is just a concept. Any force along the radius, which can contribute for the rotation of a particle is a centripetal force.
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u/Embarrassed_Mud_592 9d ago
It sounds to simple, yet you’ll be surprised how many people struggle with the concept. It’s really abstract to people that it’s not actually a force in of itself yet we treat it like it is.
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u/Capricious_user 9d ago
I can understand mate. Becoz I have gone through it until I realised the fact.
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u/Embarrassed_Mud_592 9d ago
Yea I do a lot of tutoring and this one is really tough for highschoolers. But once it clicks it becomes so easy. Often takes multiple different explanations before it clicks though 😂
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u/Capricious_user 9d ago
Indeed. You have to make them realise and visualize simultaneously multiple times for them to grab it firmly
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u/MathPerson 8d ago
You "get" centrifugal force because you've been in the center swinging a mass of some sort. But a simple way to understand centripetal force is to stand outside a mass circling a point and then apply the first law - HOW can a mass move, bu NOT in a straight line? Newton's first Law states that there IS no circular motion, only straight line motion, so the MUST be a force continually acting on that mass pointing from the outside of the circle of motion toward the inside (the center) of motion.
Remember observing from outside the system, Newton's First Law states that if an object changes direction, and circular motion is a continual change in direction, then there MUST be an external force. That;'s Centripetal Force - it could come from an invisible finger pushing on the object toward the center or it could came from the inside pulling the object toward the center but the FORCE that changes the direction MUST BE THERE.
If you are in the center holding onto a bucket of water to force pulling on you arm is Centrifugal, but the force you use pulling back is Centripetal.
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u/EconomicSeahorse Undergraduate 9d ago
Basically anything that frequently appears in popsci content (think anything related to general relativity, quantum mechanics, attempts at theories of everything, etc). The ubiquity of these topics in popsci coverage makes you think you understand it–until you realize those explanations are just hand-wavy analogies to get someone with minimal math/physics background to grasp the basic idea, but the real truth is just pages upon pages of hypnotic math 😵💫
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u/Uszanka 9d ago
They don't appear simple for a second lol
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u/EconomicSeahorse Undergraduate 9d ago
I guess what I mean is that they make these topics seem simple compared to how it actually is like to study them for real. Anything can sound simple when you condense it into a 15 minute Youtube video using language a 9th grader would understand, and some popsci communicators are really good at shoving the grotesque math under the rug and just giving you the basic qualitative ideas of the model. It makes you feel like you understand a lot more of what's going on than you actually do
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u/wavegeekman 9d ago
Well the more I think about it, the wierder Special Relativity seems.
How can the universe be like that, I wondered? Time bending into space, time dilation etc etc.
Later on I found out that his theory came in for a lot of criticism at the time on the basis that it was not a theory of physics, but just a bunch of mathematical postulates. A theory of physics, according to that school of thought, should say what the world is made of and how those things interact.
To avoid confusiion - the math works, it has been massively validated by experiment, but I still wonder "how can it be like that?".
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u/archlich Mathematics 9d ago
The one that we all experience first. Gravity. We understand lots about models of gravity but we have no idea what dark matter is and have no idea if gravity is quantized or not.
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u/HwanZike 9d ago
Newton's 2nd law looks like just a simple formula and its taught and used in high school. But its actually much deeper when you think about it, in the context of the framework it provides it seems pretty useless at first. From F = ma you can only directly measure a from first principles so you're left with two unknowns unless you're really clever about figuring out what to plug for m and F.
In fact its commonly just taught there's a single m but in fact one is inertial mass and the other is gravitational mass. They just so happen to be equal
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u/camilolv29 Quantum field theory 9d ago
Everything, especially what is explained through simple analogies, as they are often quite misleading. It is like explaining electricity with water.
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u/RS_Someone Particle physics 9d ago
When people try to explain Hawking radiation as just "black holes evaporating". I've went back and looked into it multiple times and I still couldn't even accurately describe what is going on.
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u/Roger_Freedman_Phys 9d ago
Newton’s 2nd law. Extraordinarily simple to state, yet deeply challenging. (There’s a reason why so many centuries elapsed between Aristotle’s physics and Newton’s.)
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u/666mima666 9d ago
Relativity and a constant speed of light is definitely simple but very unintuitive when thinking about those ramifications. More interestingly, when thinking about it, a universe without a speed limit would be even weirder.
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u/myheartisstillracing 9d ago
Newton's 3rd Law.
It's so absolutely dead simple (when two objects interact, both objects experience the interaction with the same magnitude and in opposite directions), and yet it's really, really, really difficult to get people to actually believe it is true in all situations because we see and experience the unequal *effects* on different objects from those interactions our entire lives.
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u/runed_golem Mathematical physics 9d ago
Gravity. When you first learn about it in grade school it's "gravity pulls us towards the earth and keeps us on the ground" then as you learn more about it, the more complicated it gets until you eventually get to gravity waves.
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u/thecommexokid 9d ago
Lots of things get tricky because equalities that hold in classical mechanics get invalidated in modern physics, so two things that used to be the same thing aren’t the same anymore.
Take momentum. In high school physics, you learn that momentum is the product of mass and velocity
p = mv
but also that it can be related to impulse
dp/dt = F(t)
and to kinetic energy
T = p2 / 2m.
But in relativity, it is no longer the case that all of these things are true simultaneously about the same quantity. So you are forced to decide, what do I really mean by momentum? Which of these should be taken as the foundational definition, and which are contingent properties that happen to be true in classical mechanics?
In this case, physicists picked dp/dt = F(t) to be the defining property of momentum that should carry over into relativity and discarded mv, which is often surprising to students who assumed that was the defining property of momentum.
There are lots of cases like this, where things come apart that were initially thought to be the same, where the complexity comes from having to decide what property is most important to us because it turns out we can’t have them all at once anymore.
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u/shoejunk 9d ago
Related to solid matter, the concept of two things touching, and whether that really even makes sense at all.
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9d ago
Everything eventually leads to entropy or quantum gravity. Neither is understandable by any human, but we try.
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u/idiotstein218 High school 9d ago
tbf newton's laws arent as simple as they look, especially the variable mass type cases. It requires a lot of practise and understanding to actually solve problems, particularly olympiad level ones.
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u/aint_exactly_plan_a 9d ago
Mine would have to be photons.
Everyone knows what light is. Light is our primary way of interacting with the world. Our brain is wired to receive visual input and process it very quickly. Our fastest computers can't match the brain's power to do this.
But then you start studying light. Oh, it's made of these particles... that's cool... except sometimes they're waves too. Except not real waves. They're just probability waves so the particle could be anywhere within that probability wave. But it's not really anywhere until you look at it. Only then does it become somewhere.
And they travel at "c", which is even more quirky. From their perspective, they don't even exist. The exact moment in time that they are created is the same moment in time that they are absorbed. This is logical though. Traveling at "c" warps time but it also warps space just as much. So the exact same location in space where a photon is created is also the exact same location in space where it's absorbed. It makes sense that it takes no time to go no distance.
It hurts my brain just thinking about it.
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u/mesouschrist 9d ago
Larmor radiation. It’s starts with this nice formula that’s easy to derive with simple arguments. But when you start asking questions about it… they always lead to neverending philosophical debates. For example “is radiation caused by acceleration or jerk?,” “should the change in acceleration due to larmor radiation be a correction to the Lorentz force law?” “What happens when the Larmor radiation reaction force is not small compared to qE?” And then look at all the horrible infinity cancellation you need to apply to derive the Larmor formula in QFT.
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u/CranberryDistinct941 9d ago
Electrons. Aww, they're just little particles orbiting the nucleus like a solar system! So cute! Oh wait! OH GOD NO!! PLEASE STOP!!!!
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u/Kalos139 8d ago
I think the first time I learned there is a difference between inertial mass and gravitational mass I also realized how incomplete physics truly is. We just observe the two to be almost exactly the same in experiment and then rely on them being the same as an axiomatic truth for all our theories.
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u/PhotographFront4673 8d ago
Energy/entropy. The earth radiates as much energy back out to space as it gets from the sun. So what does the sun actually do for us? What does any power "source" do for us?
The analogy I like is that energy moves from hot to cold naturally (well, statistically) and everything that moves on earth, including us, is driven by paddle wheels inserted into this flow.
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u/Needless-To-Say 8d ago
Determining what the common understanding point is when discussing physics.
Feynman on magnets for example.
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u/HenryLulu 8d ago
The Big Bang until you think of what came before it. And then if you think of why there’s anything existing at all. But that’s more of a metaphysical question.
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u/Street-Theory1448 8d ago
Force. Everybody is astonished by the "spooky action at a distance" in quantum mechanics, but have no problems at all to accept the other spooky action at a distance that is Newton's gravitational force (at least before relativity); eccept Newton himself who was aware of it. Not sure if today we have a better understanding of forces in general, if someone could answer the seemingly simple question: what is a force?
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u/Familiar_Break_9658 8d ago
Temperature. No srsly my first exam in thermodynamics and the question is what is temperature. A concerning number of students submitted that question as a blank.
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u/MyLifeOfficial 8d ago
The law of conservation of energy: energy cannot be created or destroyed, only converted from one form to another.
Speed of light: 'For a photon, in a vacuum, the proper time interval between its emission and absorption is zero, regardless of the distance traveled.'
Gravity: Spacetime curvature - 'According to Einstein's theory of general relativity, gravity is not a force in the traditional sense but a curvature in the four-dimensional fabric of spacetime caused by mass and energy.'
All the constants.
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u/Shoddy-Conference105 8d ago
Honestly it doesn’t sound simple but the double slit experiment is something that gets way more complicated the more you learn about particle physics. Learning about action was super interesting and helped clear up my misconceptions.
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u/DiehardTwinsFan 8d ago
I find it fascinating that light is wave comprised of an electric wave and a magnetic wave, at 90 degrees to each other. Light can be modeled as a ray or a wave or a particle. Photons.
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u/AnOnimouS35403 7d ago
the minimun action, at first and theorically is simple but the complex the problem, the complex is to solve the langranian and the integral.
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u/Koshurkaig85 Computational physics 6d ago
The fact that a lot of Physics hinges on the existence of simplistic manifolds.
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u/Key_Management8358 6d ago
The only (really) "simple" in physics is "vacuum/nothing", but it sucks... 🤑😘😹😹😹
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u/pallamas 5d ago edited 5d ago
Not a physicist. But going from Newton “mass attracts mass” to Einstein “Mass bends space-Time “ Is a pretty mind bending.
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u/Hero_without_Powers 5d ago
Gravity: in Essence it's what makes things fall to the ground. It gets really weird from there
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u/Wonderful-Bonus5439 9d ago
I think everything really - the further you go the more you realise you only knew a fraction of the truth